The present invention relates to a clock reference signal used in a global positioning system (GPS), and more particularly to a method and apparatus for calibrating a crystal oscillator.
The GPS is a satellite-based navigation system which requires a very stable local timing reference to ensure accurate navigation. There are currently three GPS systems, namely the NAVSTAR Global Positioning System controlled by the United States Defense Department, the GLONASS system maintained by the Russian Republic, and the GALILEO system proposed by Europe.
To decode these satellite signals, a GPS receiver must first acquire the signals transmitted by a minimum number of satellites. A GPS receiver required a very accurate internal frequency reference in order to lock on to the GPS signals.
In general, a GPS receiver has an internal crystal oscillator that is free running when the receiver is first turned on. Acquisition will not be successful if the difference between the PLL frequency of the GPS receiver and the GPS frequency is larger than the carrier frequency offset search range in acquisition.
Conventional GPS receivers typically use a temperature compensated crystal oscillator to provide a stable and accurate internal reference frequency. Such temperature compensated crystals are very stable with time and temperature and provide the GPS receiver with a short amount of time to acquire satellite signals. Conventional temperature compensated crystal oscillators have a frequency drift of ±1 PPM or “ppm” (parts per million) or less throughout the operating temperature range of the crystal.
However, temperature compensated crystals are very expensive. In addition, the initial acquisition time of the satellite signals may be long. A need continues to exist for a low-cost crystal oscillator for GPS receivers. The present invention provides a technical solution to overcome the frequency shift of the uncompensated crystal oscillators and simultaneously enable a fast acquisition time.